Why are there hundreds of thousands (if not millions) of species of beetles but only 23 known crocodilians? these scientists have found it has nothing to do with how old a lineage is. Instead, there are ecological limits - "adaptive zones" in time and physical habitat - where space, resources, and competition dictate how a given type of animal is more or less likely to move into new niches, which are likely to encourage the development of new species. Essentially, it doesn't take much habitat, or many individuals, to sustain a beetle species. Beetles in a place like Brazil can speciate all they want without having to have some other species vacate a habitat. Crocodiles may not have the opportunity unless an existing species dies out. This, Michael Alfaro explains, means, "If adaptive zones control biodiversity at the broadest scales, then the rate of species growth will be a good explanation of species richness only right after a lineage has entered into a new adaptive zone. Once the adaptive zone has filled up, then, no matter how much time goes by, the number of species will not change much...Most of the groups that we studied have hit their limits."
What's the implication for discovery of new species? Well, it's not a particularly earth-shaking finding that we are more likely to describe new mosquitoes than new crocodiles or mammals. It's harder to apply it to any specific habitat. There are oddities, overlaps, and evolutionary changes that create exceptions to the rules. But this is a step toward understanding why evolution has come out the way it has.